The present invention relates generally to radio frequency (RF) excited, diffusion-cooled, sealed-off CO2 lasers. The invention relates in particular to a waveguide CO2 laser including a tapered-waveguide gain-region.
There are three types of RF-exited, diffusion-cooled CO2 lasers in common use. These are the slab laser, the folded waveguide laser, and the folded, free-space-resonator laser. In a folded free-space-resonator laser, lasing modes are determined primarily by the configuration of mirrors forming the laser resonator. A slab laser includes a laser resonator in which the lasing mode or modes are constrained in one of two mutually perpendicular directions, transverse to the resonator axis, by slab-like electrodes used to excite an RF discharge in the lasing (CO2) gas. The mode shape in the other transverse direction is determined by the configuration of mirrors forming the resonator. In a folded waveguide laser, lasing modes are constrained in mutually perpendicular directions in zigzag arrangement of waveguide-channels in a dielectric slab, typically a slab of a ceramic material. The ceramic slab is bounded by electrodes for exciting an RF discharge in a lasing gas in the waveguide-channels. It is generally accepted that the power output of slab lasers scales with the discharge area for a given electrode spacing while the power out put of prior-art waveguide lasers scales with length.
Slab CO2 lasers have the highest power output capability. Slab lasers having a power output of 1000 Watts (W) are commercially available. It is generally accepted, however, that waveguide CO2 lasers have superior mode-quality to that of slab lasers and have higher efficiency. One factor contributing to the higher efficiency is diffusion cooling in both the height and width of the waveguide dimensions. One factor contributing to this superior mode-quality is the use of waveguide dimensions that constrain lasing into a single oscillation mode. The higher efficiency and superior mode-quality are presently obtained at lower output power than is available in commercial slab lasers. Waveguide CO2 lasers are commercially available with power outputs in a range between 25 W and 140 W, although waveguide lasers with power outputs up to 300 W have been custom produced for specialized applications.
FIGS. 1 and 2 schematically illustrate a prior art waveguide-block 30 of a type used in a prior-art waveguide CO2 laser. Other features of the laser such as gas containment arrangement, resonator mirrors, arrangements for sustaining an RF discharge, and cooling arrangements are omitted from FIG. 1 for convenience of illustration. Such features are well known to those skilled in the art to which the present invention pertains. A detailed description of a prior art laser including such a waveguide-block is given in U.S. Pat. No. 6,192,061 the complete disclosure of which is hereby incorporated by reference.
Waveguide-block 30 is typically formed from a ceramic material such as high density Aluminum Oxide (Al2O3) and includes two or more waveguide-channels, with 3 to 7 channels being preferred. Three waveguide-channels (waveguides) 32, 34 and 36 are depicted in FIGS. 1 and 2. Each waveguide has a height or depth H and a width W, each of which is assumed, here, to be constant. There is little freedom in varying the cross sectional dimensions H and W of a waveguide if single mode operation is desired. By way of example, dimensions of a single-mode waveguide-channel for a CO2 laser are about 0.28 centimeters (cm) high, and between about 0.28 and 0.47 cm wide.
A longitudinal resonator axis 38, folded into a Z-shape by mirrors (not shown in FIG. 1) extends through the waveguides. Waveguides 32, 34, and 36 are arranged at an angle xcex8 from each other to accommodate the folded resonator axis. Angle xcex8 is exaggerated in FIG. 1 for convenience of illustration. In practice, angle xcex8 is relatively small, for example less than about fifteen degrees (15xc2x0) with about 6xc2x0 or less being preferred. End 32B of waveguide 32 overlaps (is juxtaposed with) end 34A of waveguide 34. End 34B of waveguide 34 overlaps end 36A of waveguide 36. The degree of overlap depends on angle xcex8 and the distance at which mirrors (not shown) used to fold the resonator axis 38 are located from the ends of the waveguides. Those skilled in the art will be aware that this distance and the angle xcex8 are usually kept as small as practically possible to minimize the length and the width of the laser. The selection of the angle xcex8 is a design compromise between keeping the width of the laser small, and minimizing the waveguide overlap area. Reducing xcex8 reduces laser width, while increasing xcex8 decreases the overlap area. Reducing xcex8 also reduces the positioning sensitivity of the folding mirror for ease of resonator alignment.
Given that height H is constant, total laser power output capability provided in the uniform-width waveguides is proportional to the total area (width times length) of the waveguides. The overlapping or juxtaposition of the waveguides gives rise to common areas (AC) of the waveguides that can be considered to provide gain in only one of the waveguides or the other. Common areas AC are small compared with the total waveguide area for an angle xcex8 less than 6xc2x0. Similarly, the length of waveguide 34 can be considered to be approximately equal to the length of waveguides 32 and 36. Accordingly, the total area of the waveguides can be considered as approximately equal to the product of the number of waveguides (here, 3), the waveguide width W, and the length of any one of the waveguides. In other words, the power output of single-mode, waveguide CO2 lasers scales with the total length of the waveguides for a given width and height of the waveguide. By way of example, a total waveguide length of about 2.3 meters (m) may be required for an output power of about 150 W. A waveguide-block 30 having five folded channels providing a total waveguide length of 2.3 m may be about 47.5 centimeters (cm) long and about 7.6 cm wide.
One potential limit to the prior-art folded-resonator or folded waveguide approach to increasing total waveguide length is that, for a fixed physical length of a single waveguide, the folded waveguide-block can become as wide as it is long if the number of waveguides is increased. In addition, increasing the number of waveguides increases the number of mirrors required to fold the resonator axis to the point where alignment of the mirrors becomes very difficult. Further, as dimensions of a folded-resonator laser-package and output power increase, it becomes increasingly difficult to design uniform cooling arrangements for the laser-package that minimize temperature gradients.
Temperature gradients resulting from non-uniform cooling can cause flexing of a laser housing, resulting in beam pointing errors, among other problems. Difficulty in obtaining ceramic blocks greater than one meter in length also limits the length and thus the power scaling of CO2 waveguide lasers.
Increasing the number of waveguides increases the total area of the laser, which, in turn, increases the area of electrodes needed to maintain the RF discharge in the waveguides. As the electrode area increases, the capacitance seen by an RF power supply energizing the electrodes increases causing a decrease in impedance. The lower the impedance the more difficult it is to couple RF energy into the discharge.
Still another problem encountered in power scaling waveguide-lasers is damage to intra-resonator optical components, particularly optically coated components. In prior art CO2 waveguide lasers operated in a cavity-dumped, Q-switched, pulsed mode, for example, it is possible that intra resonator power density (power per unit area) can reach the damage threshold of intra-resonator optical components such as electro-optic switches (EO-switches) and reflective phase retarders used to implement the Q-switching and cavity dumping. Pulsed peak power-density may be on the order of several megawatts per square centimeter (MW/cm2). At this level, optical coatings on a reflective phase-retarder and anti-reflection coatings on transparent windows on the EO-switch can begin to damage after as little as 100 hours of laser operation.
There is a need to increase the power output of a single-mode waveguide CO2 laser while keeping overall dimensions comparable with prior-art, folded-resonator, waveguide CO2 laser. Preferably, this power increase should be achieved without sacrifice of reliability, mode-quality, beam pointing stability and ease of coupling RF energy into the discharge.
In one aspect, a laser in accordance with the present invention comprises a laser resonator having a resonator axis folded by mirrors into a zigzag pattern. The resonator axis extends through a plurality of waveguides. Adjacent ones of the waveguides are arranged end-to-end at an angle to each other to conform to the zigzag pattern of the resonator axis. The width of at least one of said waveguides is tapered from a narrowest width at one end thereof to a widest width at an opposite end thereof. Two or more of such tapered waveguides can be utilized.
Preferably at least one of the waveguides is selected as a mode-filtering waveguide. The mode-filtering waveguide is characterized in that it has a uniform width selected, cooperative with the length and height of the waveguide, such that laser-radiation generated in the laser resonator can oscillate in only a single oscillating mode. Two such mode-filtering waveguides may be included in the waveguide arrangement
Tapering the width of one or more of the waveguides according to the present invention allows the cross-sectional area power-saturation intensity per unit length of the resonator to increase with the resonator length. Additionally including the mode-filtering waveguide or waveguides, may provide that a single-mode folded resonator in accordance with the present invention can have a higher overall power output than a prior-art, folded, single-mode uniform-width waveguide resonator having the same number of folds and the same total (axial) length.
Various embodiments disclosed herein employ one or more uniform width waveguides and one or more tapered waveguides, combined in various orders. The specific embodiments are intended to be illustrative of some useful combinations but should not be viewed as limiting the invention to the particular configurations discussed in this summary or in the detailed description. Those skilled in the art could conceive of other combinations that those discussed herein.
In one preferred embodiment of the inventive laser, the folded resonator-axis extends through first, second, third, fourth, and fifth waveguides arranged end-to-end, in listing order, at an angle to each other to conform to the zigzag pattern. The first waveguide has a width tapering from a widest width at a first end thereof to a narrowest width at an opposite end thereof. The second and third waveguides have a first uniform width equal to the narrowest width of the first waveguide. The first uniform width is selected cooperative with the height and length of the waveguides such that the resonator will support only a single oscillating mode. The fourth waveguide has a width tapering from the first uniform width at a first end thereof to a maximum width at an opposite end. The first end of the fourth waveguide is juxtaposed with an end of the third waveguide. The fifth waveguide has a second uniform width equal to the maximum width of the fourth waveguide. The laser resonator is terminated between two resonator mirrors. One of the resonator mirrors is a plane mirror and the other is a concave mirror.
In one example of this embodiment, the laser resonator has optical components for Q-switching located on the resonator axis between said first mirror and the uniform-width fifth waveguide. The uniform width of the fifth waveguide provides that laser-radiation output is collimated. The width of the fifth waveguide is selected to reduce power density on the Q-switching components to reduce the possibility of damaging the components.
In general, for a given waveguide height (H) and gas pressure used, the power output of a tapered-waveguide laser in accordance with the present invention scales with total length of the waveguides plus the increased discharge area contributed by the tapered waveguide or waveguides. The inventive tapered-waveguide laser has a capability to extend the power output of waveguide lasers into the power range of above-discussed prior-art slab lasers, while maintaining the mode-quality associated with prior-art waveguide lasers.
In another aspect of the present invention, a laser comprises a laser resonator having a resonator axis folded by mirrors into a zigzag pattern. The resonator axis extends through at least four waveguides. Adjacent ones of the waveguides are arranged end-to-end at an angle to each other to conform to the zigzag pattern of the resonator axis. One of the mirrors is a plane mirror arranged to fold the resonator axis more than once. This can provide for simpler alignment of resonator mirrors than in an arrangement where each folding mirror folds the resonator axis only once. This simpler alignment may be enjoyed in a folded waveguide laser in which all of the waveguides have a uniform width.
In yet another aspect of the present invention, a laser comprises a laser resonator including a plurality of waveguides arranged end-to-end along a resonator axis. The resonator axis is folded by at least two mirrors into a zigzag pattern. Adjacent ones of the waveguides are arranged at an angle to each other to conform to the zigzag pattern. One of the angles between waveguides is different from another of the angles between waveguides.
In one embodiment of this unequal-angle, folded-resonator, waveguide laser there is an arrangement of two uniform-width waveguides and two tapered waveguides. The uniform-width waveguides have a width about equal to the minimum width of the tapered waveguides, and the tapered waveguides are located centrally in the arrangement with the widest ends thereof overlapping. The angle between the tapered waveguides is twice the angle between a tapered waveguide and the adjacent uniform-width waveguide. Providing the greater angle between the tapered waveguides reduces loss of potential gain due to overlapping of the waveguides at the juxtaposed ends thereof.
In still another aspect of the present invention, a laser amplifier includes a plurality of waveguides arranged end-to-end along an amplifier axis. The amplifier axis is folded by at least two mirrors into a zigzag pattern. Adjacent ones of the waveguides are arranged at an angle to each other to conform to the zigzag pattern. At least one waveguide has a width tapered from a narrowest width at one end thereof to a widest width at an opposite end thereof. Laser-radiation to be amplified enters the tapered waveguide at the narrowest end thereof and exits said tapered waveguide at the widest end thereof.
In one embodiment of the inventive amplifier, all of the waveguides are tapered from a narrowest width at one end thereof to a widest width at an opposite end thereof. The waveguides are arranged in one axial direction with the narrowest end of one thereof juxtaposed with and having the same width as the widest end of the preceding one thereof. Laser-radiation to be amplified enters each of the waveguides at the narrowest end thereof and exits at the widest end thereof.
The present invention is summarized above in terms of a number of aspects, embodiments, and advantages thereof. A detailed description of the present invention is presented hereinbelow. Those skilled in the art may recognize from this detailed description, other aspects, embodiments and advantages of the present invention without departing from the spirit and scope of the present invention.